Silicon, the second most abundant element on Earth, is widely used in the manufacture of siloxane-based semiconductors, glasses, ceramics, plastics, elastomers, resins, mesoporous molecular sieves and catalysts, optical fibers and coatings, insulators, moisture shields, photoluminescent polymers, and cosmetics [Auner, N. and Weis., J. (1998) Organosilicon Chemistry III: From Molecules to Materials, Wiley WCH; Auner, N. and Weis., J. Organosilicon Chemistry IV: From Molecules to Materials, Wiley WCH (in press); and Ball, P. (1997) Made to Measure: New Materials for the 21st Century, Princeton University Press, Princeton, N.J., USA]. The manufacture of these materials typically requires high temperatures or the use of caustic chemicals.
By contrast, the biological production of amorphous silica, the simplest siloxane [(SiO2)n], is accomplished under mild physiological conditions, producing a remarkable diversity of exquisitely structured shells, spines, fibers, and granules in many protists, diatoms, sponges, molluscs and higher plants [Simpson, T. L. and Volcani, B. E. (1981) Silicon and Siliceous Structures in Biological Systems, Springer-Verlag; and Voronkov, M. G., Zelchan, G. I. and Lukevits, E. J. (1997) Silicon and Life (2nd edn), Zinatne Publishing, Vilnius, Lithuania]. These biologically produced silicas exhibit a genetically controlled precision of nanoscale architecture that, in many cases, exceeds the capabilities of present-day human engineering. Furthermore, the biological production of siloxanes occurs on an enormous scale globally, yielding gigatons per year of silica deposits on the floor of the ocean. Diatomaceous earth (composed of the nanoporous skeletons of diatoms) is mined in great quantities from the vast primordial deposits of this biogenic silica.
Biotechnical approaches are now starting to unlock the molecular mechanisms of polysiloxane synthesis under physiological conditions, offering the prospect of new, environmentally benign routes to the synthesis and structural control of these important materials. Taking advantage of marine organisms that produce large relative masses of biogenic silica, molecular biologists have begun to isolate the genes and proteins controlling silica biosynthesis and nanofabrication.
Hildebrand and colleagues made a significant breakthrough by cloning and characterizing the cDNA encoding the first silicic-acid [Si(OH)4] transporter to be unequivocally identified [Hildebrand, M., Volcani, B. E., Gassman, W., & Schroeder, J. I. (1997) Nature 385, 688-689]. They showed, by analysis of the encoded protein and by injection of the mRNA (synthesized in vitro from the cloned cDNA) into Xenopus eggs, that the transporter protein forms a sodium-dependent transmembrane ion channel that mediates the transport of silicic acid. The action of this protein can account for the uptake of the silica precursor from the dilute pool of silicic acid in oceanic and fresh water, and similar transporters may pump silicic acid (or its conjugates) into the lumen of the silica-deposition vesicle (silicalamella), in which polycondensation (polymerization) is known to occur.
Kröger and colleagues have cloned and characterized cDNAs encoding two families of protein (at least one of which is glycosylated) that contribute to the organic sheath surrounding the silica walls of a diatom [Kröger, N., Bergsdorf, C. and Sumper, M. (1994) EMBO J. 13, 4676-4683; and Kröger, N., Lehmann, G., Rachel, R. and Sumper, M. (1997) Eur. J. Biochem. 250, 99-105.]. The proteins most intimately associated with these silica walls contain regularly repeating hydroxyl-rich domains potentially capable of interacting with the growing silica structure [Hecky, R. E., Mopper, K., Kilham, P., & Degens, E. T. (1973) Mar. Biol. 19, 323-331; Swift, D. M. & Wheeler, A. P. (1992) Phycology 28, 209-213; and Harrison, C. C. (1996) Phytochemistry 41, 37-42]. Hecky et al. had proposed that such hydroxyl-rich domains might align silicic-acid monomers, either by condensing with them (with elimination of water) to form covalent adducts or by hydrogen bonding, thus bringing them into favorable juxtaposition for their condensation to form silica. Thermodynamic calculations support the energetic feasibility of such a pathway [Lobel, K. D., West, J. K., & Hench, L. L. (1996) Mar. Biol. 126, 353-360].
Other researchers have suggested that various organic conjugates of silicic acid might serve as the proximate substrates for polymerization in vivo. Silicon catecholates have been used by Perry et al. in extensive studies of silica polymerization promoted by sugars and polysaccharides from silicified plants (Harrison, C. C. (1996) Phytochemistry 41, 37-42; Harrison, C. C., & Loton, N. J. (1995) J. Chem. Soc.-Faraday Trans. 91, 4287-4297; and Perry, C. C. & Yun, L. J. (1992) J. Chem. Soc.-Faraday Trans. 88, 2915-2921), and Mann and his colleagues recently showed that bacterial filaments can direct the deposition of a colloidal silica gel which after calcination yielded a macroporous filamentous material [Davis, S. A., Burkett, S. L., Mendelson, N. H., & Mann, S. (1997) Nature 385, 420-423].
In contrast to anthropogenic and geological syntheses of these materials that require extremes of temperature, pressure or pH, living systems produce a remarkable diversity of nanostructured silicates at ambient temperatures and pressures and at near-neutral pH. Laboratory methods have been unable to replicate these results and rely instead on extremes of pH (acid or basic) to condense silica precursors, with surfactants directing the formation of specific morphologies or patterned structures. These conditions are undesirable for environmental reasons and therefore methods to direct silica assembly under conditions similar to those used in nature (i.e. biomimetically) are desired.
Moreover, it would be desirable to extend such biomimetic methods to non-silicon products. Present methods of metal oxide fabrication for the electronics and high-tech industries require capital-intensive “fabrication-line” facilities, the use of high temperatures and high vacuum, and the costly control and remediation of strong acids, bases and other toxic and dangerous chemicals. Attempts to fabricate nanoscale metal oxide features by lithographic methods of etching or stenciling are already reaching the foreseeable limits of resolution. There is a need for an economical way of micro- and nano-fabricating metal oxides without these limitations, and without the environmental hazards of present fabrication techniques, as well as for similarly fabricating other oxides and corresponding nitrides, and their organic or hydrido conjugates and derivatives, and other related materials.
By way of further background, attention is called to the following United States Letters Patent references, each of which is distinguishable from the teachings of the present invention and from the invention in parent application Ser. No. 09/856,599.
U.S. Pat. No. 3,474,070 to Leon Levene teaches a method for hydrolyzing trifunctional organosilanes at neutral pH conditions and using an iron-containing hydrolysis catalyst, in order to produce soluble heat-curable prepolymers for the fabrication of (presumably optical) coatings. There is no contemplation of structure-direction in the formation of the ultimate coatings, i.e., there is no control of the structure of the final material by templating on the surface of the macromolecular catalyst itself. Moreover, the Levene process requires high temperature curing.
U.S. Pat. No. 4,746,693 to Martin G. Meder also teaches a method for producing a coating composition, by the aqueous emulsion hydrolysis of silanes in the presence of a nonionic catalyst and a nonionic fluorochemical surfactant. The Meder process does not take place at neutral pH, and here, too, there is no contemplation of structure-direction in the formation of the final silica films.
U.S. Pat. No. 6,004,444 to IIhan A. Aksay et al. teaches a method for transferring a microscopic pattern to a silica film during its formation through an involved process in which an acidic aqueous silica precursor solution (including a surfactant) is first “wicked” (by capillary action) into a template and then the hydrolysis reaction is accelerated through the application of an electric field. The process is carried out in contact with a flat polycrystalline substrate that also serves as a template for deposition. There is a high degree of crystallinity and mesoscopic order in the patterned silica structures formed, but this appears to be a consequence of the presence of the template and the kinetics of the hydrolysis in the presence of the heating produced by the applied electric field. The reaction conditions are acidic, not neutral, and any structure-direction is not evidently due to a mechanistic role of the catalyst, i.e. it, is the result of the “molding”, or “soft-lithography” described by Aksay et al.